Various methods are employed for different workpieces made of different materials when processing workpieces on presses. The plethora of processing methods is met with presses based on various operational principles in order to meet the particular technical requirements.
Illustratively, when deep-drawing large auto body parts using correspondingly massive tools it is desirable that the tool shall impact at minimal speed the material to be formed when the deep-drawing process begins. A so-called impact otherwise taking place between the blankholder and the drawing bottom die degrades optimal spreading of lubricant and accordingly hurts the procedure. Furthermore this impact generates considerable and undesired noise. A substantial impact will strongly load the press, the tools and the drawing system.
A hydro-mechanical hybrid drive unit is known and described in WISSENSCHAFTLICHE ZEITSCHRIFT DER TECHNISCHEN UNIVERSITAT DRESDEN, 40, 1991, vol. 3/4, where the drive unit integrates the hydraulic cylinder into the force path of a crankpress and the approximately (co)sinusoidal motion of the press plunger is complemented by an additional motion from the hydraulic cylinder. The motion of the additional hydraulic cylinder in the force path is controllable within wide limits and illustratively it is possible during deep-drawing auto body parts to gently deposit the blankholder on the drawing die at the beginning of deep-drawing.
One drawback of this known press is that substantially the complete hydraulics and control system of a hydraulic press is required in addition to the mechanical portion of the press in the force path of the press drive unit. As a result, the known press is expensive in design and manufacture.
Moreover press knuckle-joint drives are known for instance from the periodical WERKSTATT UND BETRIEB, 124, 1991, vol. 5. Such drive systems have been developed for instance to achieve a minimal and approximately constant operational speed in the vicinity of the lower reversal point of the press, as desired for instance for stamping. However the mechanical design of the knuckle-joint drive unit allows only slight latitude when kinematically designing the press drive unit.
Another drawback of the known knuckle-joint drive units is that they preclude rapid return strokes, and as a result the operational rate and hence productivity of the press are undesirably lowered.
Moreover articulating drives are known for presses, comprising a plurality of mutually articulating levers with which a uniform rotation of a drive shaft is transmitted non-uniformly to the press ram. Illustratively complex articulating drives are known from the German Auslegeschriften 1,502,326 and 2,927,503, namely in the form of a single-crank drive of a dually acting press used for deep-drawing, wherein the sheetmetal-holding ram rests in the work zone at uniform crankshaft rotation while the draw ram descends at approximately constant work speeds In the vicinity of the upper dead point the speed of both rams is much slowed, and adequate time is available to change the workpiece. The return-stroke zones remaining between the two above described regions are crossed at higher speed in order to achieve a short operational rate for one period of motion.
The known articulating or knuckle-joint drives incur the drawback that they are made up of many components and hence are complex and their manufacture is costly. Furthermore, mass production is hampered because of the number of different knuckle-joint drives. The particular design of the knuckle-joint drive directly affects the design of the remaining press.
In spite of the many known articulating or knuckle-joint drives, it is desirable in many applications to further improve the matching between the ram motion and the particulars of any one processing method. Illustratively this consideration applies to hot forging. In such an instance it is desired to achieve the highest possible ram speed at the lower dead point in order to minimize tool wear caused by tool heating in hot forging. This attempt fails with the known knuckle-joint drives because of their low rigidity. A basic drawback of the known knuckle-joint drives is that the rigidity of the knuckle-joint drive is lowered because of the large number of links within the kinematic chain of said knuckle-joint drive. As a result, a portion of the energy input made available from the knuckle-joint drive goes into press resiliency and hence is unavailable for processing the workpiece--a phenomenon which is always undesired.
A variation of the above designs is present in geared n-bar linkages involving a combination of multi-link knuckle-joint drives with at least two gears. They differ from a knuckle-joint drive preceded by a stationary gear drive in that the gears are affixed to or rest on the articulating cranks and revolve with them. Illustratively the periodical DER KONSTRUKTEUR 24, 1993, vol. 11, pp 39-40 describes such a drive unit for a draw ram of a deep-draw press.
The motion of a corresponding ram or of the ram of a cold-forming press can be generated by a geared n-bar linkage such as described in the periodical KONSTRUKTION 45, 1993, vol. 4, pp 117-120.
The known geared n-bar linkages evince the same drawbacks as the known knuckle-joint drives.
Machine components such as gears with oval or elliptical pitch curves are known. Such gears, just as eccentrically supported circular gears, are part of the non-circular gears. Non-circular gears as a rule are those with non-uniform pitch-curved gearings defined by predetermined centrodes (gear pitch curves) with pivoting or revolving output, which, on account of the toothing of the gear pitch curves, mesh together (see B SIEMON, Das exzentrisch gelagerte Zahnradpaar, dissertation, Hannover U. 1981).
The European patent document 0,254,958 A1 discloses such an intermittent drive unit employing a pair of centrally supported oval gears.
The German patent document 4,103,946 A1 describes a multi-stage drive unit with several elliptical gears that are mutually adjustable in order to vary the transmission ratio of the overall drive unit. Possible applications are blood pump drives, auxiliary drive units for internal combustion engines and for instance bicycle pedaling cranks.
Moreover a gear unit is known from the German patent document 4,337,413 C1 which comprises non-circular gears and flywheels and which besides driving a processing machine also is used especially for storing kinetic energy. The gear unit evinces an average transmission of unity and is in two stages, and besides delivering an output also acts on an additional storing shaft with a separate flywheel. The components of the transmission are so matched to one another that the kinetic energy of the overall system always is constant. If there is a change in the motion, back-impacting by the tooth sides will be prevented.
The above drive units and gearing units with non-circular gears represent special solutions foremost for the transmission of torques which are small in relation to press operations, and therefore are unsuited for pressing. Moreover such units cannot offer the transmission stages required for presses or the constrained desired ram motions.
The Japanese patent 62-114742 discloses a drive unit for a ram of a forming machine using a special non-circular pair of gears. Each of the non-circular gears is rotatably supported at its geometric center. On account of the axial symmetry of the pitch curves of the described non-circular pair of gears, the ram motion repeats identically in its forward and return motions. As a result the ram motion cannot be optimized over the total ram displacement, that is, not with respect to forming and productivity. The drawback manifests itself in that forming can only be improved in some procedures by means of the lower ram speed while other procedures requiring higher rates of forming are ignored. In addition the production rate can be increased only for a ram forming speed equal to that of conventional crank drives.